1. Field of the Invention
The invention relates in general to storage stable hydrogen electrochemical gas generating cells, and, more particularly, to improved cathode related structures for hydrogen generating cells. The invention also relates to converting commercially available Zn-air cells to efficient and storage stable hydrogen generating cells in which commercial Zn-air cells are packaged with a membrane of the type that precludes the passage of O2 and water in and out of the cell but allows hydrogen gas out of the cell. The invention further relates to a system wherein the commercial Zn-air cell is converted to a storage stable H2 gas-producing cell.
2. Background Art
Various devices have been utilized for dispensing fluids, where the fluids are dispensed over an extended period of time at a predictable substantially constant rate. One such device for dispensing fluid, as shown in FIG. 1, is based on using an electrochemical gas generating cell in which hydrogen gas is electrochemically generated to pressurize a gas chamber which, in turn, dispenses the fluid from the device.
A prior art construction of a hydrogen-generating cell is of a Zn-air type cell, shown in FIG. 2. A Zn-air cell typically utilizes zinc as the anode, a carbon based cathode and an alkaline solution as the electrolyte. The anode cap subassembly is comprised of a Zn alloy, an electrolyte, and the cap. The cathode can subassembly is comprised of a carbon-based porous electrode, a separator, and the can, all of which are crimped together using a plastic grommet as an insulator.
Various prior art patents describe the construction of such cells. For example, U.S. Pat. No. 3,894,538, issued to Richter and U.S. Pat. No. 4,023,648, issued to Orlitzky disclose metal-air cells for generating hydrogen as a motive force. Similarly, Winsel, U.S. Pat. No. 5,242,565 and Winsel EP 1013296 both disclose use of a conventional Zn-Air cell for generating hydrogen or oxygen as a motive force. However, none of these references utilize a cathode structure which is hydrogen permeable and, substantially impermeable to preclude ingress of oxygen, carbon dioxide And water (moisture) into and out of the associated cell. Although such prior art cells can be utilized as hydrogen generative cells, they are very inefficient and have short storage life in their active state mainly due to interference of O2 and CO2 as well as loss of moisture through the cathode.
Accordingly, it is an object of the present invention to provide for an improved cell construction, which overcomes the shortcomings of the prior art. It is also an object of the present invention to convert commercially available Zn-air cells and prior art hydrogen generative cells into storage stable and efficient hydrogen gas generative cells by attaching a non-porous membrane to the cathodic side of the outer housing so that O2 and CO2 are prevented from entering the cathode while water vapor is simultaneously prevented from escaping the cell through the cathode. Furthermore, it is an object of the present invention to provide a device in which hydrogen is permitted to escape from the cell.
One embodiment of the invention comprises a galvanic cell, which includes an anode cap subassembly comprising a metal anode, electrolyte, a cathode can subassembly, a micro-porous separator, and a sealing grommet. The anode may comprise zinc, lead, iron, magnesium, aluminum and mixtures and alloys thereof.
The cathode can subassembly is further comprised of a cathode that is permeable to hydrogen, but substantially impermeable to O2, H2O and CO2. In a preferred embodiment, the cathode comprises at least one of a non-porous dense electrically conducting polypropylene, a non-porous composite of carbon, PTFE (such as TEFLON((copyright)), manufactured by E. I. du Pont de Nemours and Company), and FEP foil; a palladium foil, an iron titanium foil, an iron magnesium foil, as well as metallic membranes of one or more of palladium, nickel, titanium, and, non-porous polymers, and composites of ceramics and palladium. The cathode materials will not allow O2, moisture and CO2 to permeate in and out of the cell but will allow hydrogen to escape the cell.
In another preferred embodiment, the cathode includes a graded porosity. In such an embodiment, the cathode comprises a graded porosity from a highly porous structure (50% pores with a pore size of 1 micron or greater) to a non-porous structure along its thickness. In this case, a carbon Teflon PTFE composite with graded porosity is cladded to non-porous FEP foil. This cathode structure exhibits the required properties for highly efficient hydrogen generative systems that warrant that the cathode is hydrogen-permeable but impermeable to O2, CO2, and H2O.
In another preferred embodiment, the cathode comprises a nonporous conductive cathode.
In a preferred embodiment, the cathode comprises a non-porous conductive polymer. In one such preferred embodiment, the polymer comprises at least one of conductive PTFE and conductive polypropylene or conductive polyethylene.
In a preferred embodiment, the at least one aperture of the outer shell comprises a plurality of apertures, each of which has a diameter of less than about 5 microns.
In another aspect of the invention, the invention comprises a system comprising a commercial Zn-air cell or prior art galvanic electrochemical H2 gas generating cell and a membrane. The galvanic electrochemical H2 gas-generating cell includes at least one aperture for releasing gas. The membrane is associated with the at least one aperture. The membrane is hydrogen permeable and substantially impermeable to O2, CO2 and water, to, in turn, preclude the passage of O2, CO2 and water into and out of the cell, and to facilitate the permeation of hydrogen through the at least one aperture.
The commercial Zn-air cells as well as prior art hydrogen generating electrochemical cells use a gas permeable porous cathode through which all the gases including O2, H2O, and CO2 can permeate. This permeation results in O2 and CO2 interference and water loss during operation, and in turn, low efficiency and short storage life during the hydrogen generating mode. The present embodiment of the invention describes the construction and method of converting commercial Zn-air cells and prior art hydrogen cells to more efficient hydrogen generating cells by incorporating such cells so that the cathode is not exposed to outside O2, H2O, and CO2, but allows generated hydrogen to escape.
In one preferred embodiment, the membrane is electrically conductive. In one such preferred embodiment, the membrane is selected from the group consisting of: electrically conductive non-porous polypropylene; sintered composite of carbon, PTFE, and FEP foil; palladium foil, iron titanium foil, iron magnesium foil, as well as metallic membranes of one or more of palladium, nickel, titanium, and, non-porous polymers, and composites of ceramics and palladium.
In another preferred embodiment, the membrane is electrically insulative. In one such preferred embodiment, the membrane comprises at least one of polypropylene and PTFE.
In a preferred embodiment, the system further includes an outer casing assembly encircling a portion of the membrane and at least a portion of the cell. In one such preferred embodiment, the outer casing assembly further comprises a cap, a can and an isolation grommet positioned between the can and the cap.
The present invention is also directed to a method for generating hydrogen using a zinc anode-based electrochemical cell comprising the steps of associating an electrically conductive circuit with a storage-stable hydrogen generating cell, with one end of the circuit connected to a anode subassembly of the cell, and the other end of the circuit connected to a cathode subassembly of the cell having a non-porous cathode, generating hydrogen within the cell electrochemically, and selectively releasing hydrogen from the cell through the non-porous cathode, while simultaneously preventing the passage of oxygen and water into or out of the cell.